1. Field of the Invention
This invention relates to motor operators for disconnect switches and their use in electrical transmission-distribution systems, and to an apparatus and method for sensing AC power circuit continuity in power supplies for such motor operators and the like.
2. Description of the Prior Art
Disconnect switches are a known type of electrical switchgear for assisting utilities in isolating faults in a transmission or distribution system. For example, if a circuit breaker trips, indicating a fault, disconnect switches at various locations in a power line can be operated to open prior to resetting the circuit breaker. After the breaker is reset, the switches are closed in a certain sequence to determine which section of the system is affected to cause another fault, thus locating the fault. Then the fault can be isolated with appropriate switches to allow most of the system to proceed to operate while the cause of the fault is removed.
Among present motor operators are units that contain within a housing a motor and power and control elements to operate the motor. The motor has an output mechanism for attachment to a disconnect switch for opening or closing the switch by either reciprocating or torsional motion. Such motor operators may have a radio that allows remote control of the motor and gives status information to a central control.
It is a common practice in present motor operators to receive power from the AC power line, for example by having a high voltage line transformer dropping line voltage to 120 v. AC with available power of about 60 volt-amperes, that is then applied to a battery charger including a charger transformer for reducing the voltage to about 14 v. and charging a DC battery through a charger rectifier. The charger transformer primary winding is connected to the line transformer secondary winding and a protection fuse is used ahead of the charger transformer. The battery, such as one of a nominal 12 v., 33 amp hour rating, is utilized to power not only the motor but, also, the radio and its associated Remote Terminal Unit (RTU) which processes input and output signals.
It has been a desire of certain utility companies to include a control feature in the motor operator circuitry to detect loss of AC voltage from the line. In some known equipment that provides this feature (as described further, for example, in Cleaveland/Price Inc. Bulletin DB-128A94, "Types BR, BT-D and BT-T Motor Operators for Transmission and Distribution Automation", 1994, and herein incorporated by reference for description of background to the invention), one of a number of status indications automatically reported back by radio to the utility's central control is loss of AC.
Other similar equipment (as described further, for example, in Cleaveland/Price Instruction Book IB-AA10-019D, "Auto Actuate (LVA) Device", September 1997, and herein incorporated by reference for description of background to the invention) goes beyond merely reporting a loss of AC and is used to initiate an automated operation of the motor and switch. Such a system, with what is sometimes referred to as an "Auto-Actuate" feature, involves monitoring the incoming AC voltage to the battery charger for any outage. That is done on the primary side of the charger transformer. Circuitry, such as one including digital counting and logic, is used to confirm a predetermined duration of the outage and, if confirmed, generate a signal to the motor control commanding operation of the motor and the switch. Once the switch operates, the system remains on standby until the radio receives a command to operate the switch again.
Typically, the switch is opened on a detected outage. If a breaker has tripped and was then reclosed, various open switches on the system are reclosed in sequence to locate and isolate the fault as in usual practice.
By way of further background it is noted that it is common practice to have motor operated disconnect switches on multiphase, such as three-phase, power lines with a switch for each phase. Also, motor operators can employ power supplies with some different characteristics in that some utilize the battery as a backup for AC and in some the battery DC voltage is used in conjunction with AC power to the motor. In the latter type of system there is a DC motor supplied with rectified AC power that has DC battery power between the rectified AC half cycles.
By way of further background, reference is made to U.S. Pat. No. 5,311,108, May 10, 1994, and U.S. Pat. No. 5,334,919, Aug. 2, 1994, for description of some known control systems for motor operators, including use of battery power developed from line voltage and having a radio unit; such descriptions are incorporated herein.